In some dual coil inductively coupled plasma (ICP) chambers, a plasma density may show contribution from each of the coils in the areas between the source coils (either constructive or destructive). While the process can be tuned using various techniques, there are certain fundamental signatures that result from generating plasma from conventional dual coil sources, such as the characteristic “m-shape” plasma density (and resultant on-substrate effects) observed in some processes, such as a silicon (Si) etch process for example.
Thus, the inventors have provided apparatus and methods that enable tuning of the plasma density so that issues such as the “m-shape” limitation can be addressed.